A short leg brace such as shown in FIG. 5 is conventionally known as a support used by patients who, due to paralysis resulting from cerebral hemorrhage or cerebral thrombosis or due to hemiplegia resulting from an accident, cannot move their ankle joint as they intend.
This conventional short leg brace is made of relatively rigid synthetic resin material, and comprises a foot mount 10A for supporting the foot sole S and a calf splint 20A which is to be set along the calf C, wherein the foot mount 10A and the calf splint 20A are molded integrally. With the foot mount 10A placed under the foot and the calf splint 20A set along the calf C, the short leg brace is secured by means of fixing bands 50. Thus, the short leg braces of this type stabilize the leg and the foot sole in an approximately right angle (so that the ankle joint is set at its neutral position), and prevent both plantar flexion (the flexion in the direction that the toe drops down) and dorsal flexion (the flexion in the direction that the toe is lifted up) of the ankle joint so that talipes equinus and/or drop foot are corrected.
Therefore, the short leg braces of this type fix the ankle joint relatively firmly, preventing almost any motion of the ankle joint, and thus have a drawback that they cannot allow a patient to walk in a natural and smooth manner.
It might be possible to adjust the above shown conventional short leg brace by selecting the material thereof or by adjusting the width of the junction 51 between the foot mount 10A and the calf splint 20A so as to provide some flexibility to the short leg brace, and therefore it might be possible to change the width of the junction 51 depending on the condition of the patient or on how far the patient's rehabilitation program has proceeded (by scraping the junction 51 so that its width progressively reduces as the rehabilitation program proceeds). However, because it is very difficult to restore the short leg brace if it is reduced excessively, such adjustment is seldom practiced but instead it is common to make the short leg brace anew every several years (under the current Japanese Disabled Persons Welfare Act, it is permitted to remake the short leg brace every three years).
With the ankle joint fixed by a support, it is impossible for a patient to walk in a natural manner. FIG. 6 shows one cycle of walking. The walking cycle can be divided into a stance phase in which a leg of interest (the right leg in FIG. 6) is in contact with the floor and a swing phase in which the leg of interest is apart from the floor. In the drawing, the stance phase is defined as a period between the heel contact (a) and the toe separation (1), while the swing phase is defined as a period between the toe separation (1) and the next heel contact (p).
In this walking cycle, among the ankle joint dorsal flexion muscle (precisely the tibialis anterior, hereinafter referred to as the dorsal flexion muscle) and the ankle joint plantar flexion muscle (precisely the triceps muscle of the calf, hereinafter referred to as the plantar flexion muscle), the one which is shown with a shade in FIG. 6 is the working muscle. In the period from the heel contact (a) to the foot flat (b) at which the whole foot sole comes into contact with the floor, the dorsal flexion muscle works to prevent abrupt drop of the toe. In the period from the foot flat (b) through the heel separation (h) to the toe separation (1), the plantar flexion muscle works to support the weight, move the center of gravity forward, and cause the toe to kick the floor. In the swing phase between the toe separation (1) and the next heel contact (p), the dorsal flexion muscle works to lift up the toe so that a clearance is maintained between the foot and the floor. Thus, in the walking action, the plantar and dorsal flexion muscles work alternately so that the ankle repeats the plantar and dorsal flexions.
Referring to FIG. 7, the period between the heel contact (a) and the foot flat (b) is described in more detail in the following. In this drawing, K designates the knee (or the knee joint), W the center of gravity, A the ankle (or the ankle joint), F the foot (or the part lower than the ankle), TH the thigh, and SH the shank. First, the foot F is moved forward and the heel thereof is brought into contact with the floor (this state is shown in the left-side drawings, and designated with HC which is an abbreviation of "heel contact"). Subsequently, as the body moves forward (i.e., the center of gravity moves forward), the foot is, via the state shown in the middle drawings, gradually brought into full contact with the ground until the toe contacts the floor (this state is designated with EF which is an abbreviation of "foot flat").
In the period from HC to FF, a person without a disability can control the plantar flexion of his or her ankle joint as the weight is gradually applied to the foot from the left-side state to the right-side state shown in FIG. 7(A). In other words, the dorsal flexion muscle gradually expands while preventing abrupt drop of the foot toe. As a result, the knee is substantially maintained on the line of action of.the vector representing the reaction force from the floor (the vector represents a resultant of the forces provided from the floor over the whole foot sole when the weight is applied to the foot), which is shown in the drawing by the arrow directed upward from the floor, so that the person can walk smoothly and steadily.
However, the dorsal flexion muscle of a person with hemiplegia may be weakened and the plantar flexion muscle may have an extraordinary tension, always pulling the foot toe in the direction of plantar flexion. As a result, the foot toe comes into contact with the floor immediately after HC, with the knee K positioned on a rear side with respect to the line of action of the vector representing the reaction force from the floor as shown in FIG. 7(B), leading to an overextension of the knee K. To prevent this, the short leg brace is required to have an ability to assist the dorsal flexion muscle while at the same time allowing the plantar flexion during the period from HC to FF.
In the case where the conventional short leg brace of FIG. 5 is used, because the short leg brace fixes the ankle joint in the neutral position, it is necessary to move the whole lower leg forward in order to accomplish the transition from HC to FF, as shown in FIG. 7(C). This causes the knee K to be positioned excessively on the front side of the line of action of the vector representing the reaction force from the floor, creating a problem that the knee K cannot be steadily controlled if the extensor muscle thereof is weak.
After FF in FIG. 7, the portion of the weight applied to the foot is gradually increased, causing the ankle joint A to bend in the direction of dorsal flexion, as shown by the step (c) and its subsequent steps in FIG. 6. In this period, the short leg brace is required to move freely (without a resist) in the direction of dorsal flexion so as not to hinder the natural motion of the ankle joint A. Thus, the short leg brace is required to assist the dorsal flexion muscle in the period from HC to FF ((a)-(b)) and in the swing phase ((l)-(o)) in FIG. 6. The required supplementary torque during the swing phase is very small, because during this period the short leg brace only has to lift up the foot. However, in the period from HC to FF, the required supplementary torque is relatively large because the short leg brace has to prevent abrupt drop of the foot toe against the weight increasingly applied to the foot. Thus, if the short leg brace can produce an adequate supplementary torque in this period, it will improve the user's walk over the whole walking cycle including the period after FF.
Examples of the prior art are disclosed in the specifications of Japanese Patent Publication No.61-16173 and Japanese Utility Model Publication No.61-43473.
The embodiment of No.61-16173 comprises a supporting member which extends from the foot sole to an upper part of the Achilles tendon via an outer side of the ankle. The supporting member can be secured to the leg by straps. Further, a calf splint is hinged to an upper end portion of the supporting member, and tension springs are used to urge the toe side portion toward the hinge between the supporting member and the calf splint.
However, this conventional embodiment essentially fixes the ankle joint by means of the supporting member, and the resultant essentially limited movement of the ankle joint is elastically hindered by the tension springs. Therefore, the degree of assistance to the dorsal flexion muscle and the allowable range of the plantar flexion are both quite limited and insufficient to achieve natural movement of the ankle joint.
The embodiment of No.61-43473 is provided with a large flexibility (in other words, cat be bent with a small force) as a result of greatly reducing the width of the joint 51 of the embodiment shown in FIG. 5, while a side strut having an S-shape so as to function as a spring is provided in a manner that it extends upright with its upper end vertically slideably engaged to the calf splint 20a.
In this way, with proper provision of a stopper for limiting the vertical sliding movement of the side strut, it is possible to generate a large resisting force against the plantar flexion while allowing substantially free dorsal flexion, so that an advantageous short leg brace is achieved. However, as the side strut is stretched or compressed, its degree of flexion changes and the side strut can engage and irritate the skin of the leg, creating a problem that this embodiment is not suitable for a long hour walking.
In view of such problems of the prior art, a primary object of the present invention is to provide a short leg brace which can assist the patient in achieving more natural walking. It is also contemplated that the present invention can be used in an adaptability test conducted to find a suitable supplementary torque for each patient with hemiplegia, in which the patient's walking condition is examined for various magnitudes of supplementary torque.